[LoongArch] Add definition for LVZ/LBT instructions

This patch defines the `LVZ` and `LBT` extension instructions, which
provide enough definitions for llvm-mc and llvm-objdump to correctly
handle these instructions.

It also defines the `SCR` (Scratchpad Register) register class, which
are used by the `LBT` extension instructions.

Reviewed By: SixWeining, xen0n

Differential Revision: https://reviews.llvm.org/D155917